This invention relates to polishing materials in general and more particularly to materials suitable for polishing ophthalmic lenses made from organic materials.
These polishing materials usually consist of a powdered abrasive product suspended in any suitable liquid carrier such as water or alcohol.
In practice, these abrasive products themselves are made up of one or more oxides, together with adjuvants for producing a substantial improvement in the qualities of the oxides.
The oxides most frequently used at present are as follows: aluminium or corundum oxide, stannic oxide, titanium dioxide, zirconium oxide, chromium oxide, two iron oxides (Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4), magnesium oxide or magnesia, zinc oxide and manganese oxide.
Using oxides of this kind for abrasive products, quality of the polish obtained on ophthalmic lenses made from organic materials is always relatively poor; it never approaches the quality of the polish obtained on ophthalmic lenses made from mineral materials.
Better results are now being obtained by using a rare earth metal oxide and, in particular, cerium oxide, as the abrasive product.
However, the mere use of a rare earth metal oxide in solution in water, with no other form of preparation, still only gives relatively imperfect polishing qualities, when used for polishing ophthalmic lenses made from organic materials, and numerous streaks and specks are left on the surface of the treated lenses, chiefly because of the relatively low hardness of the constituent material of the lenses.
To improve the quality of polishing, it has been proposed in German patent application No. 25 09 871 to carry out granulometric selection of the powdered cerium oxide which is to be suspended in water and to add various adjuvants, more particularly glycerine, to this suspension.
The adjuvants proposed hitherto do result in a certain improvement in the polishing qualities obtained for the surfaces treated.
However, when it comes to ophthalmic lenses made from organic materials, these polishing qualities are still not sufficiently good for it to be possible to apply subsequent surface treatments to these ophthalmic lenses under satisfactory conditions, e.g. anti-reflecting or anti-abrasion treatments, which demand that the lens has a very highly polished surface in order to have a uniform effect over the whole lens, which is, of course, absolutely essential.
Moreover, the cerium oxide-based polishing materials proposed hitherto result in excessively long polishing operations when used for polishing ophthalmic lenses made from organic materials.
Finally, these polishing materials are not stable over long periods. They are subject to settling resulting in reagglomeration, forming a solid deposit of the particles of cerium oxide which were originally in suspension, and this leads, on the one hand, to irreversible degradation of the suspension and, on the other hand, to deterioration in the quality of polishing obtained with the suspension.